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Pteraeolidia ianthina (Angas,1864)

Order: NUDIBRANCHIASuborder: AEOLIDINAFamily: Glaucidae

DISTRIBUTION

Throughout the tropical and subtropical Indo-West Pacific.

PHOTO

Upper: White juvenile animal without symbiotic zooxanthellae and larger brown animal in which symbiotic zooxanthellae are nurtured in ducts of the digestive gland both in the cerata and the body wall. Lower: Adult Pteraeolidia feeding on the solitary hydroid Ralpharia which is its preferred adult food. Coffs Harbour Region, New South Wales, Australia, December 1990. Photos: Bill Rudman.

RELATED TOPIC

Pteraeolidia ianthina, commonly known as the 'Blue Dragon' by divers in eastern Australia, is one of a group of remarkable aeolid nudibranchs which are way ahead of man in harnessing solar energy. Pteraeolidia has evolved a method of capturing and farming microscopic plants (zooxanthellae) in its own body. The plants flourish in this protected environment and as they convert the sun's energy into sugars, they pass a significant proportion on to the nudibranch for its own use. The white animal is a juvenile which as not yet developed its crop of zooxanthellae. If this species is similar to others that have been studied then it must obtain its first "injection" of zooxanthellae by feeding on a hydroid with symbiotic zooxanthellae. White juveniles are usually found in lush growths of short "turfing" hydroids, but until now no sign of zooxanthellae has been found in the hydroids. Adults can last some time without feeding, presumably obtaining sufficient nourishment from their zooxanthellae gardens. The large solitary hydroid is the preferred adult food.

This is a juvenile Pteraeolidia ianthina. As we can see from Sully Bachel's message [#22228] adults show considerable variation in colour with some having tha very white colour form as in her message while others are shades of brown and purple. The brown ones all contain specialised branches of the digestive gland which contain microscopic brown algae [see Solar powered slugs] which they apparently obtain from their food hydroids. However juveniles lack these brown plant cells and so are white.

October 29, 2008
From: Ann Clear

Hi Bill,
We have just returned from diving in the wonderful Lembeh Strait. I saw this nudibranch on my ascent, he was quite large and was moving very swiftly across the sandy bottom of a reef. I think it is of the Flabellinidae family, but have been unable to identify him. He was really lovely.

From the gemeral shape and colour of the head, this is clearly Pteraeolidia ianthina, but I must say I have never seen such a pale orange colour form like this before. As you will see from the Fact Sheet, adults usually have brown zooxanthellae in their bodies which gives them a dark brown colouration. Perhaps this one has lost its zooxanthellae?

Thanks for these photos. This colour form, with fine bluish lines, is not very common, but Chikako's photo in Nishina Masayoshi's message [#4966] from Japan shows that this colour pattern is widespread.

September 23, 2008
From: Leanne & David Atkinson

Hi Bill,
With all the discussion of Pteraeolidia ianthina recently we thought you might like to add some mature individuals from Vanuatu. These were both taken at the same dive site but on different days.

September 16, 2008
From: Nicholas Missenden

Hi Bill,

Just in relation to Erwin Kodiats' message on the rhinophores of the Pteraeolidia ianthina [#15742 ]. I have enclosed a picture that shows a very good head shot if you need one for your aeolid head Fact Sheet.

Thanks for these photos. When I see photos like this of one animal with short cerata and the other with long ones, it makes me think there must be more than one species. But I have looked at the anatomy of a whole series of animals from all parts of the world and can find no obvious differences. Perhaps DNA studies will suggest there is more than one species, but then again maybe the differences are physiological - perhaps to do with the symbiotic zooxanthellae they harbour. I have a few more messages on this species to post. I will do it today or sometime soon.

August 13, 2008
From: Roland Bircher

Dear Bill,

Recently on a dive at Bare Island I photographed what I think is a Pteraeolidia ianthina. Between the upper and lower photos it looks like the nudibranch is everting something. Is this the oral tube as you mention in an earlier answer or am I completely wrong?

As you will see from the species Fact Sheet, Pteraeolidia is a solar powered species relying on the energy produced by the microscopic plants it 'farms' in its body. The brown spotting and branching you can see in the two close-ups of the head, are clusters of these plants [zooxanthellae] in branches of the digestive system. For some reason, your animal lacks these branches of the digestive system on the left side of its face and foot, and in its left rhinophore. This could be a developmental fault or it could be that this part of the body has been damaged - perhaps bitten off - and has regrown without the digestive gland ducts.

Now that I have 'set the scene' I hope I can answer your anatomical question. The mouth is situated between the two large oral tentacles. It is a bit further back than the m I have added to the photo but if the oral tube was everted it would be over the top of the 'm'. I have put an 'f ' where something seems to be being everted. This is in fact the left side of the anterior foot. Because it is bright white, it seems to be sticking out more than on the other side, which still has the brown of the digestive system. I hope that makes sense.

Firstly I couldn't see any eggs in your second photo so I have included a close-up showing the front half of the animal a bit more clearly. This certainly falls into the group I am calling Pteraeolidia ianthina. I am not entirely convinced that all the animals you can see in the Forum messages are one species, but many years ago when I was first interested in solar powered species I dissected a whole range of different coloured animals, some with relatively short cerata like your animal and others with longer cerata, but could find no consistent differences in anatomy or radular morphology. There are names that have been given to different body shapes but there seems little point in using them unless we can define which is which.

December 5, 2006
From: Bruce Wilkie

Hi Bill,
Thought you may be interested in these photo`s of Pteraeolidia ianthina feeding on a hydroid (possibly the sea fir Solanderia fusca). I watched this animal for about 10 mins, it must have been rather hungry because it attacked the sea fir with great ferocity.

Dear Bruce,
I am definitely very interested in what this amazing solar-powered sea slug feeds on. As you will know from the Fact Sheet, the only hydroid we know Pteraeolidia feeds on is the solitary tubularian hydroid Ralpharia. However since Ralpharia doesn't have zooxanthellae its been a mystery as to where the aeolid gets its zooxanthellae from. Pteraeolidia also seems to have at least two 'forms' this very elongate form with compact ceratal groups and a shorter form with much longer cerata. So any new information on its biology is very welcome.

This hydroid certainly looks like a species of Solanderia but as far as I know Solanderia does not have symbiotic zooxanthellae in its tissues. Mind you, the very pale colour of the Pteraeolidia suggests it may be lacking zooxanthellae as well. You have added another piece to the puzzle - I can't say it has solved anything but I hope it reignites some enthusiasm for us all to try and find more about its biology.
Best wishes,
Bill Rudman

July 21, 2006
From: Charles Rowe

Hi Bill,
This is the fourth of a series of nudis from my trip to Sipadan which I would like you to identify and comment on. This nudi was tiny and we had problems focussing on it but I think I did quite a good job.

Dear Charles,
This is a solar-powered nudibranch, Pteraeolidia ianthina. You may be more familiar with it in its dark brown colour phase, but the white phase is an indication that its 'battery is flat'. If you look at the solar-powered Fact Sheet you will see that these nudibranchs store live single-celled plants [zooxanthellae] in their bodies - which gives them their brown colour - but we are not sure where they get the plants from in the first place. All juveniles are white like this, and they stay white until they start to grow zooxanthellae in their bodies.

Dear Carol,
It does indeed. Sometimes if the animal sustains an injury, then mistakes can occur during regrowth, but it looks to me that something went wrong during its development. I'll add it to the page on abnormal development.

One other interesting thing is the very pale uniform colour of this animal. Pteraeolidia keeps microscopic plants alive in its body [see solar powered sea slugs] and usually they give the animal a darker colour than this. In the close up of some of the cerata I can see no sign of the plants, which appear as small brown specks, so it is possible its pale colour is because it has lost its symbiotic plants.
Best wishes,
Bill Rudman

February 7, 2006
From: Erwin Kodiat

Hi Bill,
Just took a closer head shot of Pteraeolidia ianthina, it made me realize that the rhinophores are actually the brown one and not the bright one which I guess it is its radula, please correct me if I'm wrong.

Dear Erwin,
When I get a moment I'll do an aeolid head Fact Sheet to match the Dorid head Fact Sheet. In the meantime I have numbered the 'tentacles' in your photo.

1.rhinophores - chemosensory ['smelling' organs]2. oral tentacles - tactile [touching] organs which are perhaps also chemosensory.3. anterior foot corner - the anterior edge of the foot is sometimes rounded and sometimes has angular corners. Sometimes these corners are relatively long and tentacular, as in Pteraeolidia.

The radula is essentially an internal organ which is soemtimes partly everted like a rasping tongue during feeding. If anything is everted during feeding, it is the oral tube and part of the buccal bulb, which is the muscular structire just inside the mouth which contains the radula -see photo of Geitodoris planata [#5126] with partly everted buccal bulb.

February 2, 2006
From: Aisha. E

I'm new and never have been to this site. I'm doing a class project on the blue dragon (nudibranch). Does anybody have any information, pictures, websites, etc. Anything that you may think that might help me I would greatly appreciate.
(This animal doesn't seem to have a lot of infomation from what I've found)Aisha. E

Dear Aisha,
Have a look at my answer to an earlier question [#6999]. One problem is that common names like 'blue dragon' aren't used very widely, and even when they are they can meam different things to different people. In future use the Forum search facility. Check out the two species I mention in my earlier answer, there is quite a lot of information on both species - dont forget to look at the related messages attached at the bottom of each Fact Sheet
Best wishes,
Bill Rudman

December 13, 2005
From: Makiko Yorifuji

I'm a graduate student working on the life history of Pteraeolidia ianthina under Dr. Yoshiaki Hirano. Please give Yoshi my regards. I've just read the message about P. ianthina dated Oct. 4, 2005 [#14917 ] and I'd like to share some of my findings on this sea slug with you.

First, I've seen P. ianthina crawling or staying on/near Myrionema amboinense in the field (Okinawa, Japan). I've confirmed that P. ianthina eats M. amboinense in the laboratory. Second, I found many kinds of nematocysts probably derived from many hydroids in the cnidosacs of the slug. Prey hydroids of this slug I've confirmed so far include Tubularia, Eudendrium spp., Halocordyle disticha, in addition to M. amboinense. This slug may even eat some other cnidarians. When I gave a species of swimming anemone to hungry slugs of P. ianthina (those kept without food for about 3 months) on trial, some of them ate the sea anemone. Also I've seen a slug of P. ianthina biting a stauromedusa (Stenoscyphus inabai) in a small container in which a student was keeping these two animals together for a few hours after collecting. So, I think P. ianthina has a wide spectrum of food as an adult. Moreover, as they can survive for about a half year only with light (no prey), they may not eat Myrionema so frequently.

Besides, I'm cautious about selling and/or buying sea slugs for a personal hobby. I don't think using P. ianthina to get rid of Myrionema hydroids is a good idea.

Dear Makiko,
Thanks for this interesting information. I remember Dr Hirano mentioning he had a graduate student studying Pteraeolidia. Certainly in northern New South Wales, adult Pteraeolidia are associated with the solitary tubularian hydroid Ralpharia, but that is not to say in more tropical waters, as you have discovered, that it can potentially feed on a wider variety of cnidarians. What greatly interests me is where it gets its zooxanthellae from? At some early stage in its life cycle it must feed on a cnidarian with zooxanthellae. Have you found a potential source of zooxanthellae?

I also am uncomfortable with the idea of catching and selling Pteraeolidia as possible biological control agents in aquaria.

October 4, 2005
From: Albert B Jeffers

I have a problem with Myrionema hydroids in my reef tank. I have succcesfully controlled other organisms with the use of certain slugs, through the kind advice of you guys on this forum. I need to purchase Pteraeolidia ianthina from somewhere, I don't care where and have no problem with money when it comes to saving my extensive collection of corals.
I fully understand the hazards of keeping zooxanthellate bearing slugs in a coral tank and I am willing to pick them out after the job is done and pass them on to fellow aquarists with the same problem.
If my only option is to get them from a research facility or school, I will be glad to furnish pictures and written observation of my attempt.
Albert Jeffers (squeezix on reefcentral)
Vero Beach, Florida, USA

Dear Jeff,
I'm afraid you seem to have missed the point that most nudirbranchs have very specialised diets. Those with zooxanthellae symbiosis certainly don't eat all cnidarians with zooxanthellae, they eat only one or a few closely related species. Most zooxanthellae zymbionts get their zooxanthellae not from hard corals, but from either soft corals or sea anemones. Pteraeolidia, as you'll see from the Fact Sheet, is an exception./ It eats hydroids as an adult, but only some solitary hydroids related to Tubularia which don't have zooxanthellae. They apparently get their zooxanthellae from some unknown small hydroid as juveniles. The point I am trying to make is that there is no evidence that they eat Myrionema.

This is the reason that keeping nudibranchs in aquaria is a bad idea. Most have very specialised diets. Just because you hear that dorid nudibranchs eat sponges, it doesn't mean you can throw in any sponge and think any dorid will eat it. Similarly Pteraeolidia is so fussy we don't even know what it eats as a juvenile and as an adult it certainly doesn't eat Myrionema.
Best wishes,
Bill Rudman

September 10, 2005
From: Umut Tural

Dr. Bill Rudman

This photo was taken last year during our dive trip to Sipadan Kapalai. Among the many beautiful & interesting creatures, the only one we did very curious about it, very long worm or nudibranch, we couldnt figure out What it is. What do you think , Could be a nudibranch ? We would be very glad if you help us identification of this creature.

Dear Umut,
This spectacular worm-like animal is a nudibranch. It is the solar-powered aeolidPteraeolidia ianthina. Whether there is more than one species needs further study. I have examined animals from throughout the Indo-West Pacific and can find no anatomical difference or difference in the shape of their teeth. The only difference is that some animals are very long like this and others are much shorter, although even the relatively short ones can greatly extend in length when crawling. I must say it's a major problem when trying to post a photo of the whole animal.

Pteraeolidia ianthinais one of a variety of nudibranchs and sacoglossans which can be called 'solar-powered' because they keep plant cells, or plant organelles, alive in their tissues to photosynthesise. Have a look at the relevant Fact Sheets for more information.
Best wishes,
Bill Rudman

Dear Rainer,
Yes this is Pteraeolidia ianthina. It's a shame that because the animal is so long it is hard to post a big enough photo to show its beauty, which is in the detail. I have included a close-up alongside of the head to show some of the spectacular colouration and the way the cerata are arranged on large plate-like, rounded bases
Best wishes,
Bill Rudman

December 31, 2004
From: Toshiko Tani

Dear Dr Bill Rudman,
Thank you very much for answering my question about a creature we saw in the Beqa Lagoon, Fiji [#12851] - which you told us was Pteraeolidia ianthina! I read all of the related messages on Pteraeolidia ianthina you cited and your answer to each of the messages. Yes, they do indeed seem to vary in colors and sizes. Also thank you for educating us for how the adult forms feed themselves by hosting many microscopic plants in their bodies!! Such an interesting creature!!!

It is wonderful that such a forum on sea slugs is available on-line to gather/share the information where even an non-academic sea creature lover like myself can learn so much.

Thank you again for your kind reply to my question and I thank Australian Museum, too, for running such a good website.

P.S. I am still such a beginner diver and not relaxed enough to handle a camera in the water. The photo of the Pteraeolidia ianthina was taken by my friend Yoko who loves sea creatures, too.

Dear Toshiko,
As you will see on the Forum, there are many beautiful and interesting sea slugs in Japan, and a whole group of divers interested in them. I hope this is only the first of many messages from you
Best wishes,
Bill Rudman

December 30, 2004
From: Toshiko Tani

My friend and I saw a creature in the attached photo when we were diving near Beqa Island, Fiji, in the middle of December 2004. We went through pages of sea slug information available on the Net to identify what it was but have not had any luck. Could someone in this forum kindly tell us what this is. It may not even be a sea slug but something else...

Dear Toshiko,
This is indeed a sea slug. It is the nudibranch Pteraeolidia ianthina. As you will see on the species' Fact Sheet it is one of a very interesting group of nudibranchs which we have called solar-powered because they have many microscopic plants living in their bodies which provide them with sugars and other nutrition. Pteraeolidia is often found like this extended out in the sunlight so the plants in its body can use the sun's energy to photosynthesise. Have a look at Matthias Huerlimann's message [#12820 ] for a photo of a juvenile.

It certainly looks like a centipede, and one of the early names for this species was Flabellina scolopendrella because the French scientist who found it thought it looked like a centipede, [scolopendre = centipede in French]. Have a look at the many other messages on the page. Some of the photos will give you a good idea of the size and shape of this animal.

Dear Matthias,
Yes this is Pteraeolidia ianthina. Juveniles are white like this because they have not as yet, obtained the zooxanthellae which give them the brown colour of adults. See the Fact Sheets on solar-powered animals to see how they use the zooxanthellae.
Best wishes,
Bill Rudman

Dear Frank,This is a young Pteraeolidia ianthina. Have a look at the species' Fact Sheet where I explain how they change from white to brown as they begin to nurture single-celled plants in their bodies.Best wishesBill Rudman

January 30, 2004
From: Dana Africa

Dear SeaSlug ForumWhile scuba diving between Komodo Island and Alor Island in the Indonesian chain, I came across this nudibranch twice. Have never seen it before and cannot find it in my ID books. It is about 20 cm long. I enclose two photos - one of the whole body and one of the head.Best wishesDana Africa

Thanks Dave,Pteraeolidia has a habit of staying near its eggs. Whether this is just coincidence or a genuine brooding behaviour is not known, but if you are back at the spot in the next week or so it might be worth checkingBest wishesBill Rudman

May 24, 2003
From: Asther M. Lau

Hi again,Had found tonnes of this seaslug in East Coast of Malaysia. is it a Blue Dragon? Realised that this species likes to hang-out on wrecks and boulders. Also, it comes in brown, blue, black and even silver colors!

Dear ASther,Yes this is Pteraeolidia ianthina, often called the 'Blue Dragon'. It is one of the few examples of a common name which seems to be quite common. I don't know if you have looked at the Fact Sheet for this species. If you do you will see that it is one of the the solar-powered species which rely on sunlight to keep the plants in its body healthy.Best wishes,Bill Rudman

April 5, 2003
From: Leanne & David Atkinson

Dear Bill,After seeing Mary Jane Adams shots of Pteraeolidia ianthina mating we checked out the rest of the Pteraeolidia ianthina pages on the site. In your message with Akos' photo with eggs you talked about them laying on boulders and guarding the eggs. We recently photographed this one with eggs on a common plant/algae. Sorry about the quality of the photo, it was surgy with a sandy bottom and poor viz. The plant was swaying badly even when held. We tried hard not to disturb her/him. It was well wrapped around the eggs and difficult to get a clear shot.

Thanks Leanne & David,I've included 3 versions of your photo to show the egg ribbon in detail and the rather unusual flattened looking cerata. This species seems to be quite variable both in shape and length of the cerata. Best wishes,Bill Rudman

March 25, 2003
From: Mary Jane Adams

Hi Bill,I photographed these two Pteraeolidia ianthina mating in the Similan Islands, Andaman Sea, Thailand on March 2, 2003. The divesite is called Elephant Head Rock. The depth was 5 meters. There were numerous P.ianthina on the vertical faces of several house-size boulders on this divesite. I saw them from the intertidal region to about 10 meters. They were all the same lavender color which appears purple underwater. The smaller ones were pale pastel and the larger ones deeper shades.

Thanks Marli,These are fascinating animals. They have been observed in New South Wales, clustering in groups, and apparently 'guarding' their egg masses. They were certainly make an interesting Ph D thesis or two.Best wishes,Bill Rudman

Dear Michael,This is Pteraeolidia ianthina. It is an extremely interesting aeolid because it stores microscopic plants in its body. See the Solar powered slugs page for further information.Best wishes,Bill Rudman

July 30, 2002
From: Maliza Anuar

I've browsed around the species list and this guys looks like a Pteraeolidia ianthina. We saw quite a few of these (abt 3 per dive) in Tioman. I thought the brown coloring was kind of "boring" but after reading on how these guys are solar powered, I'll definitely be taking a closer look next time.Maliza

Dear Stewart,This is Pteraeolidia ianthina, which you will see from the two pages of messages is quite variable in shape and colour and is found throughout the tropical and subtropical Indo-West Pacific.Best wishes,Bill Rudman

October 10, 2001
From: Marli Wakeling

Hi Bill,This is a difficult one for me to figure out. This seems to possibly be some sort of Bornella, but it blended in so well with its' surroundings.Perhaps you can help solve the mystery!Location: Nusa Blowholes, Kavieng, Papua New GuineaDepth: 10 metresLength: 6 cm.Photograph: Marli Wakeling

Dear Marli,This part of the 'complex' we call Pteraeolidia ianthina. Have a look at the many messages and photos we have on this 'species'. The difference in size of cerata varies considerably and you can find all variants together. The shapes and colour forms seem to intergrade and I suspect individuals can change the shape of the cerata. I can't find any anatomical or radula differences.Cheers,Bill Rudman

Dear Nishina,Thanks to you and Chikako for these photos of the marvellous 'Solar-powered Sea Slug'Pteraeolidia. I have included an inset showing the lines of little brown specks in the cerata, which are the single-celled plants (zooxanthellae) which live in special ducts in the body wall and photosynthesise to provide nourishment for the nudibranch as well as the plants.Best wishes,Bill Rudman.

December 24, 2000
From: Yasman

Dear Bill,

This is one of the species (Pteraeolidia ianthina) I found in Karang Lebar Atoll, Thousand Islands, Indonesia. This species is quite common found in that area. It is found at depth of 40-80 feets, crawling either on sponge or sandy substrate.

Dear Yasman,Thanks for the record. It seems you have both extremes in sahpe and colour, the upper photo being a brownish animal with relatively short cerata and the lower one more bluish, with relatively long cerata. I was beginning to think the only animals you found were species of the Phyllidiidae.Bill Rudman.

Dear Akos,Far be it for to comment on your sight but there are two pages for Pteraeolidia ianthina on the Forum. Now that the Species List is getting so large a good way to find species is to use the SEARCH button at the top and bottom of every page.

The different shapes and colours found in Pteraeolidia certainly suggest the possibility of more than one species but I have looked at material from all around the Indo-West Pacific and can't find even one consistent anatomical character to separate them. One reason for the colour differences, and perhaps the length of the cerata, relates to their symbiosis with photosynthetic plant cells (zooxanthellae) which is described on Page 1 and on the Solar powered nudibranch Page. Also look at the messages on those pages and below yours on this page.

One particularly interesting phenomenon you have captured in the upper right photo is the habit of this species to cluster in groups around a collection of egg masses which I have arrowed and enlarged in the inset. All observers who have seen this phenomenon in New South Wales find them on the exposed tops of large submerged boulders in 10-15 meters. The boulders are usually covered in encrusting coralline algae (which shows there is enough light for photosynthesis) and the group of Pteraeolidia usually stay with the egg masses for at least 2-3 weeks. At this size they are feeding on the large solitary hydroid Ralpharia sp. so they must either leave the egg masses at night to feed some distance away, starve, or else rely on the products of photosynthesis.

Thanks for the interesting photos. The bottom left photo shows an intermediate sized animal by a colony of colonial hydroids on which it is possibly feeding.

March 10, 2000
From: Valda Fraser

Dear BillI have not been able to identify this nudibranch. Can you help!Locality: South Coast KwaZulu-Natal, South Africa. Pumula - near Port Shepstone - 18mDate: March 1999Size: 26mmRegardsValda Fraser

Dear Valda,This is the white form of Pteraeolidia ianthina. You will see from my comments above that this is one of the Solar-powered nudibranchs which 'farm' one-celled plants in their bodies. However they have to obtain these brownish plant cells (zooxanthellae) after they have settled out of the plankton. Before they find them they are this translucent white colour. Usually they find zooxanthellae by the tine they reach about 10-15mm in length so your animal is apparently having difficulty finding a supply of zooxanthellae. Best wishes,Bill Rudman.

Dear Clay & Patty Jo,I have a theory that the colour forms with a lot of opaque white are usually associated with white coral sand or rock. If this is so, I also think that camouflage is not the sole reason for the colour pattern. I think that as with Plakobranchus ocellatus, the white pigmentation cuts down the intensity of the sunlight reaching the endosymbiotic zooxanthellae so that their plastids are not 'burnt-out' by too much sunlight and so continue to photosynthesise. It's just my pet theory and I think it would be a good little hypothesis for a physiologist to test experimentally.

I assume the white in your animals is a skin pigment which masks the brown of the zooxanthellae, while the white of juvenile animals is caused by the absence of skin pigmentation and the absence of zooxanthellae.

One problem with the theory is that if the pigmentation is genetically controlled how do the larvae choose the right colour background when they settle? Or perhaps larvae don't travel very far and populations of white animals have gradually built up in 'white environments' - or perhaps it's all a load of rubbish!

Dear Vinka,These are all juvenile forms of Pteraeolidia ianthina. The upper photo is particularly interesting as it may be a clue to what juveniles are eating. There is a photo of an adult feeding on its preferred food, the solitary gymnoblastic tubularian hydroid Ralpharia, but despite considerable searching I have been unable to confirm what the juveniles feed on. They are often found in areas with dense growths of colonial calyptoblastic hydroids but it would make sense if they fed on gymnoblastic species. There is a special reason why it would be interesting to know what the juveniles are eating. if you look at the page on solar-powered nudibranchs you will see that adult Pteraeolidia nurtures and breeds single-celled plants (zooxanthellae) in its body, and benefits by taking some of the sugars they produce for its own nutrition. What we don't know is where it gets its first zooxanthellae from. The reason all your juveniles are white or bluish-white is that they have not yet obtained zooxanthellae. Once they do they quickly gain a brown background colour. I suspect that, like other nudibranchs with zooxanthellae, they obtain the first injection of zooxanthellae from eating a hydroid with zooxanthellae. As the large Ralpharia don't have zooxanthellae, they must get them from their juvenile food. to prove my theory I need to find the juvenile food.

Lets hope your interesting photo will lead someone to finding the answer to this puzzle.

November 20, 1999
From: Valda Fraser

Dear BillThis one has amazing elastic properties. One photo shows it relaxed and going about its own business. The other shows it disturbed. I hope you will be able to identify it. It is common in our area.Locality: South Coast KwaZulu/Natal, SOUTH AFRICA. Scottburgh, Rocky reef, 24m, November 1999. Size: 780mm

Dear Valda,This is Pteraeolidia iathina which as well as being 'elastic' is also 'solar powered', keeping single-celled zooxanthellae alive in its body where they photosynthesise and provide sugars for the nudibranch. There is some variation in shape and colour throughout the the Indo-West Pacific but I am unable to find any anatomical differences to separate them into more than one species.

August 18, 1999
From: Max Gillies

Elizabeth, Just read your post 'Pteraeolidia from Sydney' on Bill's sea slug page. great shots! I've seen this guy at Fly Point in Port Stephens (best night dive in Australia for slugs I think!) as well but didn't have a camera handy. I was interested that he was at 30m as I found mine in 8m wandering over soft coral.

I could have watched it for hours (except it was turning into a drift dive as the tide had turned).

From memory he appears in 'Sea Slugs of Western Australia' with a small amount of information. I'll bring the book in tomorrow and scan the relevant sections if you're interested.RegardsMax.

Dear Max,They are quite often found on the greyish soft-coral, Parerythropodium which forms a leathery layer over sublittoral rocks. Adult Pteraeolidia feed on the large solitary hydroid Ralpharia which is found embedded in the soft-coral colonies. There is a photo of the hydroid at the Top of the Page.

August 18, 1999
From: Betsey Hansen

Stephen,Thanks for the message. I found all of Sydney great for Sea Slugs. Unfortunately, I only spent two weeks there. I found more Sea Slugs diving there in two weeks than I have here in New York diving for ten years. I've been trying to get a transfer To Sydney to no avail.Betsey

August 8, 1999
From: Betsey Hansen

Bill,I've search you're entire slug site and can't find this one. Don't feel bad that I went through the entire slug site without finding it, though. I thoroughly enjoy going through all the slug sites and I even learn something on occasion. I promise not to call every sea slug a nudibranch anymore. I don't know if this habit is from scuba divers or Americans, but I will try to correct my friends.

This sea slug is from your part of the world. It was taken in October 1996 on the twin anchors wreck in Manly. [Sydney, New South Wales, Australia] The depth was about 30 meters and I found it crawling across the top of the wreck. It is quite beautiful, but you don't seem to have anything close to it.

Dear Betsey,It is a colour form of Pteraeolidia ianthina. Local divers call it the 'Blue Dragon' because of its likeness to the sinuous shape of tradiitonal Chinese dragons.

Pteraeolidia inathina is either quite variable in shape and colour or there are a number of similar species. Your photos are of a form which I felt was a distinct species, but after looking at the anatomy and radula of about 100 specimens from around the Indo-West Pacific, I can find no distinguishing anatomical features. It differs externally in the manner in which it holds the cerata in flattened hand-like clusters and the large whitish crescent shape mark on the base of each ceratal cluster.

Erwin,Thanks for the question. It made be realise I had never added this species into the index, although you could get to it through the Solar Power page.Yes, I'm pretty sure its a juvenile Pteraeolidia ianthina. The purple bands on the oral tentacles are fairly distinctive.Bill Rudman.